A girl with a very sexy body but an unattractive face... This phrase is generally uttered by men who examine the posterior of a lady as they drive by, On the approach they say "Oooohhh" and if her face does not match the beauty of her posterior as they pass they will say "Naaaaaa", hence Una.
Someone doing something that they shouldn't be doing e.g. a ref giving a bad decision at a game, a child playing with a box of matches, a garda pulling up someone for pissing in a dark alley etc.